In conventional cellular and PCS (personal communications system) wireless systems, signals transmitted from a base station (cell site) to a user (remoter terminal) are usually received via an omni-directional antenna; often in the form of a stub antenna. These systems often sacrifice bandwidth to obtain better area coverage, stemming from the result of less than desirable signal propagation characteristics. For instance, the bit (binary digit) to Hz ratio of the typical digital Cellular or PCS system is often less than 0.5. Lower binary signal modulation types, such as BPSK (Binary Phase Shift Keying) are used, since the effective SNR (Signal to Noise Ratio) or C/I (Carrier to Interference Ratio) are often as low as 20 dB. In fact, for voice based signaling, the threshold C/I (or S/N) ratio for adequate quality reception of the signal is about 17 dB.
For wireless systems directed towards data applications, it is desirable to significantly increase the SNR or C/I in order to employ higher order (binary) modulation techniques, such as QAM-64 (Quadrature Amplitude Modulation, with 64 points in the complex constellation). These higher order modulation schemes require substantially greater C/I (or SNR) thresholds; typically higher than 26 dB. For the case of MMDS (multi-user multipath distribution system) signals, where the carrier frequencies are higher (around 2500 MHz), the propagation characteristics are even worse. There is a need therefore for transmission systems that can both satisfy the coverage (propagation) demands, as well as generate high C/I or SNR levels.
One option is to increase the size of the terminal equipment (TE), or remote, antenna gain. This requires increasing the size. Additionally, it helps to increase the elevation (i.e., vertical height above ground level) of the antenna. The higher you place an antenna, the better the system gain. For a simple planar earth model, the total system path loss (attenuation) is a function of each (transmit and receive) antenna's directive gain (towards one another). However, this path loss is also a function of the height (from ground level) of each antenna. Thus, as you increase the height, from ground, the total system path loss decreases, which is an increase in the overall system link performance, or system gain. The link performance (system) gain increases 6 dB every time you double one of the antenna's height from the ground level. If you double both (i.e., transmitting and receiving) antennas' heights, the total gain (link performance) goes up by 12 dB (6 dB+6 dB). Therefore, doubling the height from the ground is equivalent to quadrupling the size (area) of the antenna; which produces 4.times. (or 6 dB) of directive gain.
In conventional analog MMDS systems, this (i.e., increase of SNR or C/I) has been traditionally accomplished by installing a large reflector type antenna (with up to 30 dBi of directional gain) on a rooftop, or a pole. The disadvantages are a complex, difficult, and costly installation; as well as poor aesthetics.
The migration of the MMDS frequency spectrum, from an analog video system, to a wireless data and Internet system, demands a more user friendly (easier) installation method, with much lower cost. The difficulty here is designing a system with sufficient directional gain, as to overcome loss with transmission through walls, as well as being easy to install, and orient; by the consumer, or other persons without specialized skills.